Programmed material consolidation methods for fabricating heat sinks

ABSTRACT

Programmed material consolidation processes for fabricating heat sinks include the selective consolidation of previously unconsolidated material. The heat dissipation element of the heat sink that has been fabricated by such processes can have non-linear or convoluted passageways therethrough to enhance air flow. An optical recognition system may be used in conjunction with programmed material consolidation processes to ensure that a heat sink is fabricated or positioned on the appropriate location of an electronic component, such as a semiconductor device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/449,219,filed May 29, 2003, now U.S. Pat. No. 7,026,191, issued Apr. 11, 2006,which is a divisional of application Ser. No. 09/502,107, filed Feb. 10,2000, now U.S. Pat. No. 6,730,998, issued May 4, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to heat sinks used to dissipateheat from semiconductor devices during normal operation thereof.Particularly, the present invention pertains to the use ofstereolithographic techniques to fabricate heat sinks for use onsemiconductor devices, to heat sinks so fabricated, and to semiconductordevices including stereolithographically fabricated heat sinks.

2. State of the Art

Heat Sinks

During normal use, semiconductor devices generate heat. Adequatedissipation of the heat generated during normal use of a semiconductordevice is necessary for the safe and reliable operation of an electronicappliance that includes the semiconductor device. If the semiconductordevice reaches an excessively high temperature, the integrated circuitsof the semiconductor device can fail or a circuit board fire can result,damaging the electronic system of which the semiconductor device is apart.

While some semiconductor devices are able to dissipate sufficientamounts of heat without an additional heat sink or heat spreader, stateof the art semiconductor devices with increased speed, circuitcomplexity, and circuit density often require added heat sinks.

In particular, as semiconductor devices have become more dense in termsof electrical power consumption per unit volume, heat generation hasgreatly increased, requiring package construction which dissipates thegenerated heat much more rapidly. As the state of the art progresses,the ability to adequately dissipate heat is often a severe constraint onthe size, speed, and power consumption of an integrated circuit design.

In this application, a heat sink will be distinguished from a “heatspreader,” the former pertaining to a structure with a heat transferportion or element positioned proximate to a semiconductor device and aheat dissipation portion or element relatively more remote from thesemiconductor device, the latter pertaining to a member which channelsheat from a semiconductor die to leads which exit the die package.However, a heat sink and a heat spreader may together be used to cool adevice.

Typically, heat sinks are fabricated from materials with good thermalconductivity, such as metals (e.g., aluminum, copper alloys, etc.),ceramic materials, and glass. The heat transfer portion of a heat sinkis configured to absorb heat from the semiconductor device proximatethereto and, therefore, generally contours to at least a portion of asurface of the semiconductor device. The heat dissipation portion of aheat sink may include a series of small protrusions, which are typicallyreferred to as “fins,” that receive heat from the heat transfer portionof the heat sink and are configured to dissipate the heat away from thesemiconductor device as air flows between the fins. The shapes, sizes,arrangement, spacing, and numbers of fins on a heat sink are configuredso as to optimize the heat dissipation capabilities of the heat sinkwith respect to the particular heat dissipation needs of a specific typeof semiconductor device.

Heat sinks are typically manufactured separately from the semiconductordevices to which they are subsequently secured.

Conventionally, metal heat sinks have been manufactured by extrusion orcasting processes. When extruded, molten metal is forced through anextrusion die to produce an elongated extrusion of a cross-section takentransverse to the length thereof of a desired heat sink configuration.The elongated extrusion is then sectioned transverse to the lengththereof to provide the heat sinks. Cast heat sinks are manufactured bydisposing a molten quantity of heat conductive material into arefractory mold.

Heat sinks can also be machined from blocks of material. As conventionalheat sinks have spaced apart fins, however, machining processes waste aconsiderable amount of material. In addition, due to the small size andhigh complexity of conventional heat sinks, the use of machiningprocesses can be very time consuming and expensive. For these reasons,the use of machining processes to manufacture heat sinks is somewhatundesirable.

The use of extrusion, casting, and machining processes to manufactureheat sinks are also somewhat undesirable since each of the processeslimit the possible configurations of the manufactured heat sinks. Forexample, when extrusion is used, the transverse cross-section takenalong the entire length of each heat sink has the same two-dimensionalshape, being that imparted by the two-dimensional configuration of theextrusion die. When heat sinks are cast, the configurations thereof aredetermined by the casting molds. Typically, molds have two parts, andmay include additional inserts to facilitate the formation of morecomplex features. State-of-the-art machining processes are limited to,at most, seven axes. Typically, however, less complex three-axis orfive-axis machines are used. Nonetheless, certain types of features,such as internally confined cavities and non-linear channels cannot beformed easily when casting or state-of-the-art machining equipment isused.

An alternative method for manufacturing heat sinks is disclosed in U.S.Pat. No. 5,814,536, issued to Rostoker et al. on Sep. 29, 1998(hereinafter “the '536 Patent”). The '536 Patent discloses the use ofpowder metallurgy techniques to form a heat sink. Thus, the heat sink isformed from a mixture of powdered metal (e.g., copper, aluminum,tungsten, titanium, and alloys thereof) and a suitable binder. Themixture is placed into a mold, where the metal particles are bonded toadjacent particles, or sintered together, under appropriate pressure andat an appropriate temperature. The binder, if any, is removed (i.e.,burned off) during the sintering process. The sintered heat sink canthen be machined to provide features that may not be readily obtained orpossible to obtain by the sintering process alone. Since the sinteringprocess of the '536 Patent employs a mold, it is somewhat undesirabledue to the previously mentioned conformational limitations that arepresent when a mold is used.

As noted above, a prefabricated heat sink is conventionally assembledwith a semiconductor device. The assembly can then be packaged by knowntechniques, such as by transfer molding of a particle-filled polymer, asknown in the art. If such an assembly is packaged, however, thepackaging mold must usually be configured so as to receive at least aportion of the heat sink to permit its projection beyond the polymerpackaging. The manufacture of molds configured to receive heat sinks issomewhat undesirable due to the complexity of the mold designs and thehigh costs of machining such molds.

The art does not teach a method of fabricating heat sinks onsemiconductor devices or of fabricating heat sinks by stereolithography,or layered manufacturing, processes.

Stereolithography

In the past decade, a manufacturing technique termed“stereolithography,” also known as “layered manufacturing,” has evolvedto a degree where it is employed in many industries.

Essentially, stereolithography as conventionally practiced involvesutilizing a computer to generate a three-dimensional (3-D) mathematicalsimulation or model of an object to be fabricated, such generationusually effected with 3-D computer-aided design (CAD) software. Themodel or simulation is mathematically separated or “sliced” into a largenumber of relatively thin, parallel, usually vertically superimposedlayers, each layer having defined boundaries and other featuresassociated with the model (and thus the actual object to be fabricated)at the level of that layer within the exterior boundaries of the object.A complete assembly or stack of all of the layers defines the entireobject, and surface resolution of the object is, in part, dependent uponthe thickness of the layers.

The mathematical simulation or model is then employed to generate anactual object by building the object, layer by superimposed layer. Awide variety of approaches to stereolithography by different companieshas resulted in techniques for fabrication of objects from both metallicand non-metallic materials. Regardless of the material employed tofabricate an object, stereolithographic techniques usually involvedisposition of a layer of unconsolidated or unfixed materialcorresponding to each layer within the object boundaries, followed byselective consolidation or fixation of the material to at least apartially consolidated, fixed, or semisolid state in those areas of agiven layer corresponding to portions of the object, the consolidated orfixed material also at that time being substantially concurrently bondedto a lower layer of the object to be fabricated. The unconsolidatedmaterial employed to build an object may be supplied in particulate orliquid form, and the material itself may be consolidated or fixed, or aseparate binder material may be employed to bond material particles toone another and to those of a previously-formed layer. In someinstances, thin sheets of material may be superimposed to build anobject, each sheet being fixed to a next lower sheet and unwantedportions of each sheet removed, a stack of such sheets defining thecompleted object. When particulate materials are employed, resolution ofobject surfaces is highly dependent upon particle size, whereas when aliquid is employed, surface resolution is highly dependent upon theminimum surface area of the liquid which can be fixed and the minimumthickness of a layer that can be generated. Of course, in either case,resolution and accuracy of object reproduction from the CAD file is alsodependent upon the ability of the apparatus used to fix the material toprecisely track the mathematical instructions indicating solid areas andboundaries for each layer of material. Toward that end, and dependingupon the layer being fixed, various fixation approaches have beenemployed, including particle bombardment (electron beams), disposing abinder or other fixative (such as by ink-jet printing techniques), orirradiation using heat or specific wavelength ranges.

An early application of stereolithography was to enable rapidfabrication of molds and prototypes of objects from CAD files. Thus,either male or female forms on which mold material might be disposedmight be rapidly generated. Prototypes of objects might be built toverify the accuracy of the CAD file defining the object and to detectany design deficiencies and possible fabrication problems before adesign was committed to large-scale production.

In more recent years, stereolithography has been employed to develop andrefine object designs in relatively inexpensive materials, and has alsobeen used to fabricate small quantities of objects where the cost ofconventional fabrication techniques is prohibitive for same, such as inthe case of plastic objects conventionally formed by injection molding.It is also known to employ stereolithography in the custom fabricationof products generally built in small quantities or where a productdesign is rendered only once. Finally, it has been appreciated in someindustries that stereolithography provides a capability to fabricateproducts, such as those including closed interior chambers or convolutedpassageways, which cannot be fabricated satisfactorily usingconventional manufacturing techniques. It has also been recognized insome industries that a stereolithographic object or component may beformed or built around another, pre-existing object or component tocreate a larger product.

However, to the inventor's knowledge, stereolithography has yet to beapplied to mass production of articles in volumes of thousands ormillions, or employed to produce, augment or enhance products includingother, pre-existing components in large quantities, where minutecomponent sizes are involved, and where extremely high resolution and ahigh degree of reproducibility of results is required. In particular,the inventor is not aware of the use of stereolithography to fabricateheat sinks for use with semiconductor devices. Furthermore, conventionalstereolithography apparatus and methods fail to address the difficultiesof precisely locating and orienting a number of pre-existing componentsfor stereolithographic application of material thereto without the useof mechanical alignment techniques or to otherwise assuring precise,repeatable placement of components.

SUMMARY OF THE INVENTION

According to one aspect, the present invention includes a method forfabricating heat sinks for use with semiconductor devices. In apreferred embodiment of the method, a computer-controlled, 3-D CADinitiated process known as “stereolithography” or “layeredmanufacturing” is used to fabricate the heat sinks. Whenstereolithographic processes are employed, a heat sink is formed as aseries of superimposed, contiguous, mutually adhered layers of material.

As it is important that heat sinks absorb heat from a proximatesemiconductor device and dissipate the heat, the heat sinks of thepresent invention are preferably manufactured from materials that aregood heat conductors. Accordingly, the stereolithography processes thatare preferred for fabricating the heat sinks of the present inventionare capable of fabricating structures from materials with good thermalconductivity.

In one such stereolithography process, known as “selective lasersintering” or “SLS,” structures are fabricated from layers of powderedor particulate material. The particles in selected regions of each ofthe layers can be bonded together by use of a laser under the control ofa computer. The laser either heats the material particles and sintersadjacent particles together, heats a binder material mixed in with theparticles to bond the particles, or heats a binder material with whichthe material particles are coated to secure adjacent particles in theselected regions of a layer to one another.

Another exemplary stereolithography process that may be used tofabricate heat sinks incorporating teachings of the present invention isreferred to as “laminated object manufacturing” or “LOM.” Laminatedobject manufacturing involves the use of a laser or other cutting deviceto define the peripheries of a layer of an object from a sheet ofmaterial. Adjacent layers of the object are secured to one another toform the object.

The stereolithographic heat sink fabrication method of the presentinvention preferably includes the use of a machine vision system tolocate the semiconductor devices or substrates upon which heat sinks areto be fabricated, as well as the features or other components on orassociated with the semiconductor devices or substrates (e.g., bondwires, leads, etc.). The use of a machine vision system directs thealignment of a stereolithography system with each semiconductor deviceor substrate for material disposition purposes. Accordingly, thesemiconductor devices or substrates need not be precisely mechanicallyaligned with any component of the stereolithography system to practicethe stereolithographic embodiment of the method of the presentinvention.

In a preferred embodiment, the heat sink to be fabricated upon asemiconductor device component in accordance with the invention isfabricated using precisely focused electromagnetic radiation in the formof a laser under control of a computer and responsive to input from amachine vision system, such as a pattern recognition system, to defineeach layer of the object to be formed from a layer of material disposedon the semiconductor device or substrate.

According to another aspect, the present invention includesstereolithographically fabricated heat sinks, as well as semiconductordevices that include stereolithographically fabricated heat sinks. Asstereolithographic processes are used to fabricate these heat sinks, theheat sinks may be formed with features that cannot be defined by use ofconventional extrusion, sintering, or machining processes.

Other features and advantages of the present invention will becomeapparent to those of skill in the art through consideration of theensuing description, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a side view of a semiconductor device with a heat sinkembodying teachings of the present invention secured to a surfacethereof;

FIG. 2 is a cross-section taken along line 2—2 of FIG. 1;

FIG. 3 is a side view of a semiconductor device with another heat sinkembodying teachings of the present invention secured to a surfacethereof;

FIG. 4 is a cross-section taken along line 4—4 of FIG. 3;

FIG. 5 is a top view of the semiconductor device shown in FIGS. 3 and 4;

FIG. 6 is a partial perspective view of a semiconductor wafer withunsingulated semiconductor devices having heat sinks fabricated on thebacksides thereof;

FIG. 7 is a schematic representation of an exemplary stereolithographyapparatus, a selective laser sintering apparatus, that can be employedin the method of the present invention to fabricate heat sinks onsemiconductor devices or other substrates in accordance with the methodof the present invention;

FIG. 8 is a schematic representation of another exemplarystereolithographic apparatus, a laminated object manufacturingapparatus, that can be employed in the method of the present inventionto fabricate heat sinks in accordance with the method of the presentinvention;

FIG. 9 is a partial cross-sectional side view of a semiconductor deviceor substrate disposed on a platform of a stereolithographic apparatusand depicting a heat sink being fabricated on the semiconductor deviceor substrate; and

FIG. 10 is a cross-sectional view of another embodiment of a heat sinkaccording to the present invention, depicting the heat sink disposedadjacent a surface of a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION Heat Sinks

According to one aspect, the present invention includes heat sinks andassemblies including a semiconductor device and a heat sink. The heatsinks of the present invention are stereolithographically fabricated, orlayer-manufactured. Thus, heat sinks incorporating teachings of thepresent invention have a plurality of superimposed, contiguous, mutuallyadhered layers of heat conductive material. Moreover, since layeredmanufacturing processes can be used to fabricate features, such asinternally confined cavities and non-linear or convoluted passageways,that cannot be fabricated by use of other processes, heat sinksincorporating teachings of the present invention can include suchfeatures. FIGS. 1–5 illustrate exemplary configurations of heat sinksincorporating teachings of the present invention.

With reference to FIG. 1, an assembly is shown that includes asemiconductor device 10 and a heat sink 20 incorporating teachings ofthe present invention. As illustrated, semiconductor device 10 is aflip-chip type semiconductor device, such as a flip-chip die or ballgrid array package, with conductive structures 16 protruding from anactive surface 12 thereof. A thin layer 18 of thermally conductiveadhesive material, such as a suitable epoxy, is disposed on an opposite,backside 14 of semiconductor device 10. The adhesive material of layer18 preferably withstands high temperatures, such as those that willoccur during normal operation of semiconductor device 10.

Layer 18 secures a heat transfer element 22 of heat sink 20 proximatebackside 14. Heat transfer element 22 is configured to transfer heatthat is generated during use of semiconductor device 10 away fromsemiconductor device 10. Accordingly, it is preferred that layer 18 beformed from a material that will readily conduct heat.

As illustrated, heat transfer element 22 of heat sink 20 has channels 24extending therethrough. Channels 24 are configured to permit air to flowthrough heat transfer element 22 and to thereby facilitate cooling ofsemiconductor device 10 as the air flowing through channels 24 carriesheat away, or dissipates heat, from heat transfer element 22. As shownin FIG. 2, channels 24 may be non-linear or convoluted. Channels 24 arepreferably configured so as to facilitate the desired amount of air flowthrough heat transfer element 22 of heat sink 20 and, thus, tofacilitate a desirable level of heat dissipation away from semiconductordevice 10.

Heat sink 20 also has a heat dissipation element 26 adjacent heattransfer element 22, opposite semiconductor device 10. Heat dissipationelement 26 includes several upwardly extending fins 28. Fins 28 arespaced apart so as to permit air to flow therebetween and, thus, todissipate heat away from semiconductor device 10.

FIGS. 3–5 illustrate an assembly that includes a semiconductor device 10and another embodiment of a heat sink 20′ incorporating teachings of thepresent invention. Heat sink 20′ includes solid heat transfer element22′ and a heat dissipation element 26′ adjacent heat transfer element22′, opposite semiconductor device 10.

Heat dissipation element 26′ includes two sets of fins 30 and 32. Fins30 are linear and protrude upwardly from heat transfer element 22′. Fins32 are spaced apart and positioned substantially concentrically relativeto each other. As shown in FIGS. 3 and 4, spaces 33 between adjacentfins 32 are non-linear or convoluted passageways through which air canflow. Each fin 32 has an upwardly protruding region 34, a bend 36, and alaterally extending region 38.

Turning now to FIG. 10, another embodiment of a heat sink 40 accordingto the present invention is illustrated. Heat sink 40 has a heattransfer element 42 and a heat dissipation element 44. A receptacle 46formed in heat transfer element 42 is configured to receive at least aportion of a semiconductor device 50. As illustrated, receptacle 46receives a backside 52 and a lower portion of the periphery 54 ofsemiconductor device 50. Receptacle 46 conforms to a portion of thesurface of semiconductor device 50 and contacts the entire backside 52,as well as a portion of the periphery 54 thereof to cup semiconductordevice 50 to facilitate the transfer of heat therefrom to heat sink 40.Heat dissipation element 44, which is remote from semiconductor device50, has spaced apart fins 48 extending therefrom.

Methods of Fabricating Heat Sinks

In another aspect, the present invention includes methods of fabricatingheat sinks according to the present invention, such as those illustratedin and described with reference to FIGS. 1–5.

Turning now to FIG. 6, heat sinks 20 according to the present inventioncan be assembled with or fabricated on backsides 14 of semiconductordevices 10, such as bare or minimally packaged semiconductor dice, whilesemiconductor devices 10 are still part of a wafer 72. Eachsemiconductor device 10 on wafer 72 is separated from adjacentsemiconductor devices 10 by a street 74.

While the heat sink fabrication process of the present invention ispreferably performed substantially simultaneously on severalsemiconductor devices or other substrates, such as prior to singulatingsemiconductor devices 10 from wafer 72 or on a collection of individualsemiconductor devices or other substrates, such as partial wafers,individual semiconductor devices or other substrates can also beprovided with heat sinks in accordance with teachings of the presentinvention. As another alternative, the method of the present inventioncan be used to substantially simultaneously fabricate heat sinks 20 on acollection of different types of semiconductor devices or othersubstrates.

The heat sinks of the present invention are preferably fabricated from athermally conductive material, such as copper, aluminum, tungsten,titanium, or a ceramic material. By way of example and not to limit thescope of the present invention, the heat sinks can be manufactured fromthermally conductive materials in powdered or particulate form or in theform of thin sheets.

For simplicity, the ensuing description is limited to an explanation ofa method of stereolithographically fabricating heat sinks 20 directly onsemiconductor devices 10 having bare backsides 14. As should beappreciated by those of skill in the art, however, the method describedherein is also useful for fabricating heat sinks separately from asemiconductor device or other substrate, as well as for disposing heatsinks on packaged semiconductor devices or semiconductor devices havingone or more layers of protective material on the backsides thereof.However, the effectiveness of heat transfer from a packaged or coateddevice will naturally be somewhat compromised.

Stereolithography Apparatus and Methods

FIG. 7 schematically depicts various components, and operation, of anexemplary stereolithography apparatus 80 to facilitate the reader'sunderstanding of the technology employed in implementation of the methodof the present invention, although those of ordinary skill in the artwill understand and appreciate that apparatus of other designs andmanufacture may be employed in practicing the method of the presentinvention. The preferred, basic stereolithography apparatus forimplementation of the method of the present invention, as well asoperation of such apparatus, are described in great detail in UnitedStates Patents assigned to DTM Corporation or to Board of Regents, TheUniversity of Texas System, both of Austin, Tex., or to The B.F.Goodrich Company of Akron, Ohio, such patents including, withoutlimitation, U.S. Pat. Nos. 4,863,538; 4,944,817; 5,017,753; 5,132,143;5,155,321; 5,155,324; 5,156,697; 5,182,170; 5,252,264; 5,284,695;5,304,329; 5,316,580; 5,332,051; 5,342,919; 5,352,405; 5,385,780;5,430,666; 5,527,877; 5,648,450; 5,673,258; 5,733,497; 5,749,041; and5,817,206. The disclosure of each of the foregoing patents is herebyincorporated herein by this reference.

With continued reference to FIG. 7 and as noted above, a 3-D CADdrawing, in the form of a data file, of an object (e.g., heat sink 20 ofFIGS. 1 and 2) to be fabricated is placed in the memory of a computer 82controlling the operation of apparatus 80, if computer 82 is not a CADcomputer in which the original object design is effected. In otherwords, an object design may be effected in a first computer in anengineering or research facility and the data files transferred via wideor local area network, tape, disc, CD-ROM, or otherwise as known in theart to computer 82 of apparatus 80 for object fabrication.

The data is preferably formatted in an STL (for STereoLithography) file,STL being a standardized format employed by a majority of manufacturersof stereolithography equipment. Fortunately, the format has been adoptedfor use in many solid-modeling CAD programs, so translation from anotherinternal geometric database format is often unnecessary. In an STL file,the boundary surfaces of an object are defined as a mesh ofinterconnected triangles.

Data from the STL files resident in computer 82 is manipulated to buildan object, such as a heat sink 20, illustrated in FIGS. 1 and 2, onelayer at a time. Accordingly, the data mathematically representing oneor more objects to be fabricated are divided into subsets, each subsetrepresenting a slice or layer of the object. The division of data iseffected by mathematically sectioning the 3-D CAD model into at leastone layer, a single layer or a “stack” of such layers representing theobject. Each slice may be from about 0.003 to about 0.020 inch thick. Asmentioned previously, a thinner slice promotes higher resolution byenabling better reproduction of fine vertical surface features of theobject or objects to be fabricated.

Apparatus 80 includes a horizontal platform 90 on which an object is tobe fabricated or a substrate disposed for fabrication of an objectthereon. Platform 90 is preferably vertically movable in fine,repeatable increments responsive to computer 82. Material 86 is disposedin a substantially uniform layer of desired thickness by a particulatespreader that operates under control of computer 82. The particulatespreader includes two cartridges 104 a and 104 b disposed adjacentplatform 90 and a roller or scraper bar or blade 102 that is verticallyfixed and horizontally movable across platform 90. As a sufficientquantity of particulate material 86 to form a layer of desired thicknessis pushed upward out of each cartridge 104 a, 104 b by a verticallymovable support 106 a, 106 b, respectively, roller or scraper bar orblade 102 spreads that quantity of particulate material 86 in a uniformlayer of desired thickness (e.g., 0.003 to 0.020 inches) over platform90, a substrate disposed thereon, or an object being fabricated onplatform 90 or a substrate thereon. Supports 106 a, 106 b of cartridges104 a, 104 b are preferably vertically movable in fine, repeatableincrements under control of computer 82.

By way of example and not limitation, and as noted above, the layerthickness of material 86 to be formed, for purposes of the invention,may be on the order of about 0.003 to 0.020 inch, with a high degree ofuniformity. It should be noted that different material layers may havedifferent heights, so as to form a structure of a precise, intendedtotal height or to provide different material thicknesses for differentportions of the structure.

With continuing reference to FIG. 7, in a selective laser sinteringembodiment of the heat sink fabrication process of the presentinvention, material 86 preferably comprises resin-coated particles ofone or more thermally conductive materials, such as copper, aluminum,tungsten, titanium, ceramics, or a mixture of any of the foregoing,which material 86 is deposited by cartridges 104 a, 104 b and roller orscraper bar or blade 102 over platform 90 with the latter in itsuppermost position. Alternatively, the particles of thermally conductivematerial may be uncoated, and used alone or mixed with particles of asuitable binder resin.

A fixative head, such as a laser 92, an ink jet nozzle, or a metal spraygun, is suspended above platform 90. The type of fixative head employeddepends upon the nature of the particulate material 86 employed tofabricate the object, as well as an optional binder employed toconsolidate particles of material 86 in selected regions of the layer.

When the fixative head includes a laser 92, apparatus 80 may alsoinclude a galvanometer 94 with one or more pivotal mirrors. Beforefabrication of a first layer of an object is commenced, the operationalparameters for apparatus 80 are set to adjust the size (diameter, ifcircular) of the laser light beam 98 used to consolidate or fix material86. In addition, computer 82 automatically checks and, if necessary,adjusts by means known in the art the surface level 88 of material 86over platform 90 or a substrate upon which an object is to be fabricatedto maintain same at an appropriate focal length for laser beam 98.Alternatively, the height of the mirror of galvanometer 94 may beadjusted responsive to a detected surface level 88 to cause the focalpoint of laser beam 98 to be located precisely at the surface ofmaterial 86 at surface level 88 if level 88 is permitted to vary,although this approach is more complex.

The size of the laser beam “spot” impinging on the surface of material86 to consolidate or fix same may be on the order of 0.001 inch to 0.008inch. Resolution is preferably ±0.0003 inch in the X-Y plane (parallelto surface 100) over at least a 0.5 inch×0.25 inch field from a centerpoint, permitting a high resolution scan effectively across a 1.0inch×0.5 inch area. Of course, it is desirable to have substantiallythis high a resolution across the entirety of surface 100 of platform 90to be scanned by laser beam 98, such area being termed the “field ofexposure,” such area being substantially coextensive with the visionfield of a machine vision system employed in the apparatus of theinvention as explained in more detail below. The longer and moreeffectively vertical the path of laser beam 96/98, the greater theachievable resolution.

The sequence of operation and movements of platform 90, cartridges 104a, 104 b and their supports 106 a, 106 b, roller or scraper bar or blade102, and laser 92 or another type of fixative head are controlled bycomputer 82.

Once roller or scraper bar or blade 102 spreads and smooths material 86into a first thin layer 108 of substantially uniform thickness (forexample, 0.003 to 0.020 inch) over platform 90 or a substrate disposedthereon, laser 92 directs a laser beam 96 toward galvanometer 94-mountedmirrors, which reflect a laser beam 98 toward selected regions of layer108 in order to affix the particles of material 86 in the selectedregions by melting or sintering particles of the thermally conductivecomponent of material 86 or by melting a binder component of material 86to secure adjacent particles of the thermally conductive component ofmaterial 86 that are exposed to laser beam 98 to one another. Particlesof material 86 in these selected regions of layer 108 are preferablyaffixed in a regular horizontal pattern representative of a first orlowermost transverse layer or slice of the object to be fabricated, asnumerically defined and stored in computer 82. Accordingly, laser beam98 is directed to impinge on particle layer 108 in those areas where thecorresponding layer of the object to be fabricated is comprised of solidmaterial and avoids those areas outside of a periphery of thecorresponding layer of the object to be fabricated, as well as thoseareas of the corresponding layer where a void or aperture exists. Laser98 is withdrawn upon consolidation of material 86 in regions comprisingat least the peripheral outline of the corresponding layer of the objectbeing fabricated.

With reference to FIG. 9, when material 86 in each of the regions oflayer 108 that correspond to solid areas of the corresponding layer ofthe object to be fabricated have been exposed to laser beam 98, a firstparticle layer 110, or first preform layer, is formed. First particlelayer 110 has at least the peripheral outline of the corresponding layerof the object being fabricated at that vertical or longitudinal level,material 86 within apertures or voids in layer 110 remainingunconsolidated as loose, unfused particles.

Next, platform 90 is indexed downwardly a vertical distance which may ormay not be equal to the thickness of the just-fabricated layer 110 a(i.e., a layer-manufactured structure may have layers of differentthicknesses). Another layer 110 b of unconsolidated particulate material86 is then formed over layer 110 a as previously described. Laser beam98 is then again directed toward selected regions of the new layer 110 bto follow a horizontal pattern representative of a next, higher layer orslice of the object to be fabricated, as numerically defined and storedin computer 82. As each successive layer 110 is formed by consolidatingmaterial 86 in selected regions, the consolidated material is preferablyalso secured to the immediately underlying, previously fabricated layer110 a. It will be appreciated that, in FIG. 9, the thicknesses of eachlayer 110 has been exaggerated to clearly illustrate the layeredmanufacturing process.

Of course, since an object to be fabricated by use of astereolithography apparatus, such as apparatus 80, may not haveuniformly configured and sized cross-sections taken transverse to thelength thereof, adjacent layers or slices of the object, whilecontiguous, may not be identical.

The deposition and smoothing of layers 108 of unconsolidated particlesof material 86 and the selective fusing of particles of material 86 inselected regions of each successive layer 108 is continued under controlof computer 82 for hundreds or even thousands of layers until arecognizable three-dimensional structure gradually emerges, and thelayering process is further continued until a completed object has beenfabricated. At any time during the fabrication process, or thereafter,unconsolidated particulate material 86 is removed and may be recovered.Any recovered material may be subsequently used to form another object.

As an alternative to the use of a laser to sinter or otherwise bondparticles of material 86 in the selected regions of each layer 108together to form layers 110, an ink jet nozzle or a metal spray gun maybe employed as the fixative head. Such a fixative head deposits a liquidbinder (e.g., resin or metal) over the particles of material 86 inselected regions of each layer 108, penetrating therebetween andsolidifying, thus bonding particles in the selected regions of layer 108to at least partially consolidated regions of the next underlying formedlayer 110. If an ink jet nozzle is employed as the fixative head, thebinder may comprise a non-metallic binder such as a polymer compound.Alternatively, when a metal spray gun is used as the fixative head, ametallic binder such as a copper or zinc alloy or Kirksite, aproprietary alloy available through Industrial Modern Pattern and MoldCorp., may be employed. In the case of a metal alloy, the binder may besupplied in wire form which is liquified (as by electric arc heating)and sprayed onto the uppermost particulate layer. Another alternative isto liquify the distal end of the binder wire with a laser or otherheating means immediately above the unconsolidated powder layer ratherthan using a metal spray.

FIG. 8 illustrates a laminated object manufacturing (LOM) variation ofthe heat sink fabrication process of the present invention. LOM employssheets of material to form an object. As depicted in FIG. 8, anapparatus 200 for effecting the LOM method includes a platform 202,actuating means 204 for moving platform 202 in vertical increments, asheet feeder 206, a laser head 208, and a control computer 210. Sheetfeeder 206 may comprise a photocopier-type feeder and provide individualsheets, or may comprise a roll-type feeder with a feed roller and atake-up roller, as desired. In either case, a sheet 212 of suitablematerial, such as a thin metal (e.g., copper, aluminum, tungsten,titanium, etc.) or a ceramic or glass sheet, is placed on platform 202.Laser head 208, under control computer 210, cuts an outline of theperiphery of that layer of the object being fabricated. The surroundingsheet material may then be removed, if desired, and a second, uncutsheet 212′ placed over sheet 212 is bonded to sheet 212 by suitablemeans, after which laser head 208 cuts the perimeter outline of thesecond layer of the object. If desired, laser head 208 may be used torapidly heat the second sheet 212′ and bond it to the first sheet 212before second sheet 212′ is cut at its periphery. Alternatively, aheated roller 214 may be biased against and rolled over the uppermostsheet 212′ to secure the uppermost sheet 212′ and the immediatelyadjacent, underlying sheet 212 to each other before the uppermost sheet212′ is cut to define the periphery of the corresponding layer of theobject being fabricated. The embodiment of FIG. 8 is particularlysuitable for substantially concurrently forming a large plurality ofheat sinks on the backside of an unsingulated semiconductor wafer orother large-scale substrate.

Such bonding can be effected by melting or sintering, or by an adhesivematerial disposed on the top, bottom, or both surfaces of each sheet.One or both surfaces of the sheets may be pre-coated with adhesive, oradhesive may be applied thereto, such as by rolling or spraying, duringthe layered manufacturing process.

Referring again to FIG. 7, in practicing the present invention, acommercially available stereolithography apparatus operating generallyin the manner as that described above with respect to apparatus 80 ispreferably employed, but with further additions and modifications ashereinafter described for practicing the method of the presentinvention. For example and not by way of limitation, the SINTERSTATION®2000, SINTERSTATION® 2500, and SINTERSTATION® 2500 plusstereolithography systems, each offered by DTM Corporation of Austin,Tex., are suitable for modification.

It should be noted that apparatus 80 useful in the method of the presentinvention includes a camera 140 which is in communication with computer82 and preferably located, as shown, in close proximity to galvanometer94 located above surface 100 of support platform 90. Camera 140 may beany one of a number of commercially available cameras, such ascapacitive-coupled discharge (CCD) cameras available from a number ofvendors. Suitable circuitry as required for adapting the output ofcamera 140 for use by computer 82 may be incorporated in a board 142installed in computer 82, which is programmed as known in the art torespond to images generated by camera 140 and processed by board 142.Camera 140 and board 142 may together comprise a so-called “machinevision system” and, specifically, a “pattern recognition system” (PRS),operation of which will be described briefly below for a betterunderstanding of the present invention. Alternatively, a self-containedmachine vision system available from a commercial vendor of suchequipment may be employed. For example, and without limitation, suchsystems are available from Cognex Corporation of Natick, Mass. Forexample, the apparatus of the Cognex BGA Inspection Package™ or the SMDPlacement Guidance Package™ may be adapted to the present invention,although it is believed that the MVS-8000™ product family and theCheckpoint® product line, the latter employed in combination with CognexPatMax™ software, may be especially suitable for use in the presentinvention.

It is noted that a variety of machine vision systems are in existence,examples of which and their various structures and uses are described,without limitation, in U.S. Pat. Nos. 4,526,646; 4,543,659; 4,736,437;4,899,921; 5,059,559; 5,113,565; 5,145,099; 5,238,174; 5,463,227;5,288,698; 5,471,310; 5,506,684; 5,516,023; 5,516,026; and 5,644,245.The disclosure of each of the immediately foregoing patents is herebyincorporated herein by this reference.

Of course, apparatus 200 depicted in FIG. 8 could also be equipped withsuch a machine vision system.

Stereolithographic Fabrication of the Heat Sinks

Referring now to FIGS. 7 and 9, in order to facilitate fabrication ofone or more heat sinks 20 in accordance with the method of the presentinvention with apparatus 80, a data file representative of the size,configuration, thickness and surface topography of, for example, aparticular type and design of semiconductor device 10 or other substrateupon which one or more heat sinks 20 are to be fabricated is placed inthe memory of computer 82. Also, it may be desirable to place a datafile representative of the various features of semiconductor device 10in memory.

One or more semiconductor devices 10, wafers 72, or other substrates maybe placed on surface 100 of platform 90 to have heat sinks 20 fabricatedthereon. Camera 140 is then activated to locate the position andorientation of each semiconductor device 10, including those on a wafer72, or other substrate. The features of each semiconductor device 10,wafer 72, or other substrate are compared with those in the data fileresiding in memory, the locational and orientational data for eachsemiconductor device 10, wafer 72, or other substrate then also beingstored in memory. It should be noted that the data file representing thedesign size, shape and topography for each semiconductor device 10 orother substrate may be used at this juncture to detect physicallydefective or damaged semiconductor devices 10 or other substrates priorto fabricating a heat sink 20 thereon or before conducting furtherprocessing or assembly of semiconductor device 10 or other substrates.Accordingly, such damaged or defective semiconductor devices 10 or othersubstrates can be deleted from the stereolithographic heat sinkfabrication process, from further processing, from further testing, orfrom assembly with other components. It should also be noted that datafiles for more than one type (size, thickness, configuration, surfacetopography) of semiconductor device 10 or other substrate may be placedin computer memory and computer 82 programmed to recognize not only thelocations and orientations of each semiconductor device 10 or othersubstrate, but also the type of semiconductor device 10 or othersubstrate at each location upon platform 90 so that material 86 may beat least partially consolidated by laser beam 98 in the correct patternand to the height required to fabricate heat sinks 20 in theappropriate, desired locations on each semiconductor device 10 or othersubstrate.

Continuing with reference to FIGS. 7 and 9, a substantially uniformlayer 108 of material 86 is disposed over wafer 72 or the one or moresemiconductor devices 10 or other substrates on platform 90 to a depthsubstantially equal to the desired thickness of a formed layer 110 ofheat sink 20.

Laser 92 is then activated and scanned to direct beam 98, under controlof computer 82, toward specific locations of surface 88 relative to eachsemiconductor device 10 or other substrate to effect the aforementionedpartial cure of material 86 to form a first layer 110 a of each heatsink 20. Platform 90 is then lowered and another layer 108 of material86 of a desired thickness disposed over formed layer 110 a. Laser 92 isagain activated to add another layer 110 b to each heat sink 20 underconstruction. This sequence continues, layer by layer, until each of thelayers 110 of each heat sink 20 have been completed.

In FIG. 9, the first, bottommost layer of heat sink 20 is identified bynumeral 110 a, and the second layer is identified by numeral 110 b. Asillustrated, heat sink 20 has only a few layers 110. In practice of theinvention, however, heat sinks 20 will often have many thin layers 110.Accordingly, heat sinks 20 with any number of layers 110 are within thescope of the present invention.

Each layer 110 of heat sink 20 maybe built by first defining anyinternal and external object boundaries of that layer with laser beam98, then hatching solid areas of that layer of heat sink 20 locatedwithin the object boundaries with laser beam 98. An internal boundary ofa layer may comprise a portion of a channel 24 (see FIGS. 1 and 2), aspace 33 between adjacent fins 32 (see FIGS. 3–5), a through-hole, avoid, or a recess in heat sink 20, for example. If a particular layerincludes a boundary of a void in the object above or below that layer,then laser beam 98 is scanned in a series of closely-spaced, parallelvectors so as to develop a continuous surface, or skin, with improvedstrength and resolution. The time it takes to form each layer 110depends upon the geometry thereof, the surface tension and viscosity ofmaterial 86, and the thickness of that layer.

Once heat sinks 20 have been fabricated, platform 90 is elevated andremoved from apparatus 80, along with any substrate (e.g., semiconductordevice 10, wafer 72 (see FIG. 6), or other substrate) disposed thereonand any stereolithographically fabricated structures, such as heat sink20. Excess, unconsolidated material 86 (e.g., excess powder orparticles) may be manually removed from platform 90, from any substratedisposed thereon, and from heat sink 20. Each semiconductor device 10,wafer 72, or other substrate is removed from platform 90.

Residual particles of the thermally conductive material that was used tofabricate heat sink 20 are preferably removed by use of known solventsor other cleaners that will not substantially degrade, deform, or damageheat sink 20 or the substrate (e.g., semiconductor device 10) on whichheat sink 20 was fabricated. Such cleaning is particularly importantwhen electrically conductive materials, such as copper, aluminum,tungsten, or titanium, are used to fabricate heat sink 20, as a residueof such electrically conductive materials can cause electrical shortsthat will result in failure of semiconductor device 10.

Although FIGS. 7–9 illustrate the stereolithographic fabrication of heatsink 20 on a substrate, such as a semiconductor device 10, a wafer 72,or another substrate, heat sink 20 can be fabricated separately from asubstrate, then secured thereto by known processes, such as by the useof a suitable adhesive material.

The use of a stereolithographic process as exemplified above tofabricate heat sink 20 is particularly advantageous since a large numberof heat sinks 20 may be fabricated in a short time, the dimensions andpositions thereof are computer controlled to be extremely precise,wastage of construction material 86 is minimal, and thestereolithography method requires minimal handling of semiconductordevices 10, wafers 72, or other substrates.

Stereolithography is also an advantageous method of fabricating heatsinks according to the present invention since, when resinous bindersare used to secure adjacent particles of thermally conductive materialin selected regions, stereolithography can be conducted at substantiallyambient temperature, the small spot size and rapid traverse of laserbeam 98 resulting in negligible thermal stress upon semiconductordevices 10, wafers 72, or other substrates, as well as on the featuresthereof.

The stereolithography fabrication process may also advantageously beconducted at the wafer level or on multiple substrates, savingfabrication time and expense. As the stereolithography method of thepresent invention recognizes specific semiconductor devices 10 or othersubstrates, variations between individual substrates are accommodated.Accordingly, when the stereolithography method of the present inventionis employed, heat sinks 20 can be simultaneously fabricated on differenttypes of semiconductor devices 10 or other substrates, as well as onboth semiconductor devices 10 and other substrates.

Stereolithography may also be used to form a wafer-level array of heatsinks separately from a semiconductor wafer, each heat sink of the arraycorresponding to a semiconductor device of the wafer. These heat sinkscan be bonded to a wafer, then the wafer separately singulated with theheat sinks being simultaneously singulated.

While the present invention has been disclosed in terms of certainpreferred embodiments, those of ordinary skill in the art will recognizeand appreciate that the invention is not so limited. Additions,deletions and modifications to the disclosed embodiments may be effectedwithout departing from the scope of the invention as claimed herein.Similarly, features from one embodiment may be combined with those ofanother while remaining within the scope of the invention.

1. A method for fabricating a heat sink configured to be assembled witha semiconductor device, comprising: providing thermally conductivematerial in an unconsolidated state; and defining at least a portion ofa heat sink from the thermally conductive material.
 2. The method ofclaim 1, wherein providing thermally conductive material comprisesproviding particulate material.
 3. The method of claim 1, whereindefining comprises selectively consolidating the thermally conductivematerial to an at least partially consolidated state.
 4. The method ofclaim 3, wherein selectively consolidating is effected in accordancewith a program.
 5. The method of claim 4, wherein selectivelyconsolidating comprises stereolithography.
 6. The method of claim 1,wherein providing comprises providing a first layer comprising thethermally conductive material.
 7. The method of claim 6, whereindefining comprises defining at least boundaries of a corresponding,first layer of the heat sink in the first layer of thermally conductivematerial.
 8. The method of claim 7, further comprising: superimposing atleast one additional layer of thermally conductive material over thefirst layer of thermally conductive material; defining at leastboundaries of at least one additional, corresponding layer of the heatsink in the at least one additional layer of thermally conductivematerial; and securing the at least one additional layer of the heatsink to the first layer of the heat sink.
 9. The method of claim 7,wherein providing the first layer comprising the thermally conductivematerial comprises providing a layer comprising particulate material.10. The method of claim 9, wherein defining at least boundaries of thecorresponding, first layer of the heat sink comprises consolidatingparticles located in regions of the first layer of thermally conductivematerial that correspond to at least boundaries of the first layer ofthe heat sink.
 11. The method of claim 10, wherein consolidating alsoincludes consolidating particles located in regions of the first layerof thermally conductive material that correspond to solid regions of thefirst layer of the heat sink.
 12. The method of claim 9, whereinconsolidating comprises sintering adjacent particles in the regions ofthe first layer of thermally conductive material that correspond to atleast boundaries of the first layer of the heat sink.
 13. The method ofclaim 9, wherein consolidating comprises securing adjacent particles inthe regions of the first layer of thermally conductive material thatcorrespond to at least boundaries of the first layer of the heat sinkwith a binder material.
 14. A method for fabricating heat sinks,comprising: providing a fabrication substrate carrying a plurality ofunsingulated semiconductor devices; fabricating heat sinks on at leastsome of the semiconductor devices; and separating at least some of thesemiconductor devices from the fabrication substrate.
 15. The method ofclaim 14, wherein fabricating heat sinks comprises: providing thermallyconductive material in an unconsolidated state over a surface of thefabrication substrate; and defining at least a portion of each of theheat sinks from the thermally conductive material.
 16. The method ofclaim 15, wherein defining comprises selectively defining at least theportion of each of the heat sinks.
 17. The method of claim 16, whereinselectively defining is effected under control of a program.
 18. Amethod of fabricating a semiconductor device component, comprising:positioning at least one substrate on a support surface; recognizing alocation and orientation of the surface of the at least one substrate;and selectively fabricating at least a portion of a heat sink proximatethe surface of the at least one substrate, the heat sink comprising atleast one layer of at least partially consolidated material.
 19. Themethod of claim 18, wherein selectively fabricating is effected inaccordance with a program.
 20. The method of claim 18, furthercomprising: storing data including at least one physical parameter ofthe at least one substrate and of the heat sink in computer memory, andusing the stored data in conjunction with a machine vision system torecognize the location and orientation of the at least one substrate.21. The method of claim 20, further comprising: using the stored data,in conjunction with the machine vision system, to effect the selectivelyfabricating.
 22. The method of claim 18, further including securing theat least one substrate to the support surface.